1. Field of the Invention
The present invention relates to a recording and reproducing disc driving apparatus.
2. Discussion of the Background
FIG. 1 is an exploded perspective view showing an important part of a conventional magnetically recording and reproducing disc driving apparatus. In FIG. 1, a reference numeral 1 designates a main base having a top surface area of the same dimension as the projection of a main body of the conventional recording and reproducing disc driving apparatus. The main base 1 supports a disc driving motor 2 (shown by a dotted line) for rotating the magnetic disc medium, a stepping motor 3 for moving a head (not shown), a sensor 4 such as a TK00 sensor for determining the position of the head, and so on. Both ends of a guide rod 6 are fixed to the main base 1 by means of two pairs of clamps 7 and bolts 8. A head carriage 5 is slidably mounted on the guide rod 6. The head carriage 5 has an extension 5a which is engaged with a screw shaft 3a connected to the stepping motor 3 so that the head carriage 5 is stepwisely moved by the rotation of the stepping motor 3.
In recent years, the recording and reproducing disc driving apparatuses have become more and more small and thin, and at the same time, the capacity of the memory has become large. On the other hand, the magnetic characteristics of magnetic disc media have improved to provide high performance and high density. With the improvement of magnetic disc media, demands on the recording and reproducing disc driving apparatus having a high track pitch and highly accurate dimensions have increased, while low manufacturing cost is required.
The magnetic disc medium is prepared by coating a magnetic layer at a thickness of about 1mm-1.3mm on the surface of a base film. The magnetic disc medium expands and contracts due to change in temperature and moisture. In order to follow such change, the main base 1 as an element of the recording and reproducing disc driving apparatus has been formed by die-casting aluminum and has been subject to fine processing. (The aluminum die-case material has substantially same thermal expansion coefficient as the magnetic disc medium.) Accordingly, the manufacturing cost for the main base is very high. Further, the disc driving motor 2 should be accurate so as to rotate the magnetic disc medium without misalignment of the axial center. Similarly, selection of material and improvement in accuracy of machining should be considered for the stepping motor 3, the head carriage 5 and the guide rod 6, which are elements that determine the position of the head.
In the conventional recording and reproducing disc driving apparatus having the construction as above-mentioned, the main base 1 is formed such that its top surface has substantially the same dimensions as the surface area of the conventional apparatus when it is viewed from the top. Accordingly, it has been difficult to manufacture the main base of a small size, small weight and at a low manufacturing cost. The conventional main base has a rectangular shape, and the size of the top surface is substantially the same as the size of the recording and reproducing disc driving unit when viewed from the top. The main base is similar to the main frame 22 disclosed in FIGS. 1 and 3 of U.S. Pat. No. 4,205,355, the housing 32 as shown in FIG. 5 of U.S. Pat. No. 4,415,940, and the chassis 103 as shown in FIGS. 9 and 10 of U.S. Pat. No. 4,417,289. Namely, the main base has a one-piece structure including element bearing parts as well as portions unnecessary to support the elements. Accordingly, it has been difficult to form a main frame of a small size and weight.
Further, major elements constituting a disc driving unit are mounted on the main base 1 which has been finely finished. As described before, since the dimension in projection from the top of the main base 1 is substantially the same as that of the main body of the recording and reproducing disc driving apparatus, the weight of the main base is greater. It has been not easy to handle and carry, and much time is needed for assembling work.